Go to System Preferences -> Keyboard
Tick Show Keyboard & Character Viewers in menu bar
From the Menu, select Show Keyboard Viewer, play with the option key and you will see how to obtain any character.
For most of the non-spanish keyboards option key and e will with you the accent ´ then press the vowel you want to accentuate.
option e then a = á
option e then e = é
option e then i = í
option e then o = ó
option e then u = ú
option n then n = ñ